This invention relates generally to thickened aqueous polymer compositions and methods for thickening aqueous polymer compositions. In particular, this invention describes a method for controlling viscosity drift for thickened aqueous compositions comprising a pigment and a copolymer containing polymerized units of phosphate- or phosphonate-containing monomers, and aqueous compositions made thereby that show minimal viscosity drift.
Aqueous polymer systems, for example aqueous coatings compositions comprising emulsion polymer binders, typically use thickeners to obtain the desired degree of viscosity needed for the proper formulation and application of the aqueous system. One general type of thickener used in aqueous polymer systems is referred to in the art by the term “associative.” Associative thickeners are hydrophobically modified so as to provide an aqueous system with certain characteristics such as ease of formulation, pigment settling prevention, film build during application, spatter resistance, low sag, good flow, and leveling. These associative thickeners may have hydrophobic groups attached to or within the water-soluble polymeric backbone. The polymeric backbone may be constructed from one or more blocks of polymerized oxyalkylene units, typically polyethylene oxide or polypropylene oxide. They may also utilize a cellulosic backbone.
Most of the associative thickeners presently on the market are sold as pourable aqueous liquids. For ease of use, it is desirable for the viscosity (Brookfield at 6 rpm) of such thickener products to be less than 15,000 centipoise (cps), or even less than 5,000 cps, so that the product will readily drain from its storage container, and be readily incorporated into the aqueous system to which it is added. The viscosity of the aqueous thickener product can be decreased by reducing the active solids concentration, but this has the drawback of limiting formulation latitude in terms of weight solids of the aqueous system to be thickened by the product.
Many aqueous compositions of polymeric binders comprising polymerized units of phosphate- and phosphonate-containing monomers have proven to be particularly difficult to thicken, since these compositions suffer from a significant viscosity increase (viscosity drift) occurring from a period of hours to a period of weeks after formulating and addition of thickener. This is problematic to commercial formulators and remains as a roadblock on the route to commercialization of such phosphorus containing polymers, the latter being considered desirable for providing improved titanium dioxide utilization efficiency and improved hiding. That is, phosphorus containing binders have been shown to improve the spacing between titanium dioxide pigment particles in the dried coating film.
United States Patent Application Publication No. US 2007/0021536 A1 describes an aqueous composition containing pigment and an acrylic polymer and having reduced flocculation of pigment particles. Table 3 (paragraph 0029) illustrates the problem of viscosity drift for thickened aqueous compositions comprising a pigment and a copolymer containing polymerized units of phosphate- or phosphonate-containing monomers. The problem is alleviated to some extent by the addition of at least one compound containing a pyrophosphate linkage and having no more than 12% carbon. Despite this disclosure, other methods to minimize viscosity drift are desired. Particularly desirable is a thickener that can be used to increase viscosity of these aqueous polymer compositions to a target viscosity (KU), and which resists viscosity drift, thereby eliminating the relatively expensive additive.
A need in the art remains, therefore, for pourable associative thickeners with both low viscosity and the highest active thickener solids possible, which provide thickened systems with minimal viscosity drift for aqueous compositions comprising polymers with phosphorus containing monomers.